Meph is currently still a research chemical in most countries, however it has been made illegal in some parts of Europe down to its similarities to MDMA. Like MDMA, it can be snorted or bombed. It gives users quite a strong buzz, making it a ever increasingly popular club drug.

Ollie: Mate Ive got some more mephedrone in for the weekend.
Jack: Sweet as a nut son, sort us out a gram.

4-MethylMethCathinone, chemical compound analogue of Cathinone. Cathinone being the "natural" amphetamine found in Khat plant. In England were it was most popular, it fell out side of any laws. In other countries, it was illegal before it came out due to analogue laws (i.e. USA, and France). And it was banned in other countries after some accidents (deaths, injuries) were related to the compound.
It could be bought on the internet as a plant feeder, with a starting price towards £10 per gram ($15; 11€) but if you bought 20g, the price per gram was towards £7 ($10.7; 7.8€).
In the same way as MDMA, it causes the release of Serotonin in the brain, thus creating the euphoria, and sometimes depression after the come down.
Mephedrone is, since the 29/03/2010, on route to be banned in the UK, because of irresponsible users. Youngsters misunderstood the words "legal high", believing that it was a "safe high", in doing so, Overdoses were common place. Also, the media published every death related to Mephedrone, as they did for ecstasy just before it's ban. Note: the media reports 1/250 deaths from paracetamol or aspirin but 1/1 deaths from Meph.
Also to note: there are more death from alcohol every day then from mephedrone.